It has been known that a surface of powder is coated with another substance to improve the properties of the powder and to provide various properties with the powder, and there are increasing demands for obtaining powder having peculiar properties. In particular, such powder is demanded that has a complex function including properties inherent to metallic powder or metallic compound powder with other properties combined therewith. In order to produce the powder, it has been considered that plural layers of metallic oxides with uniform thickness are formed on base particles.
The inventors have found such a process that a metallic film is formed on base particles, and the powder is whitened by the reflection effect of the film (JP-A-3-271376 and JP-A-3-274278), and such a process that base particles are dispersed in a metallic alkoxide solution, and a uniform metallic oxide film having a thickness of from 0.01 to 20 μm is formed on the surface of the base particles by hydrolyzing the metallic alkoxide, so as to form powder having a metallic oxide film containing a heterogeneous metal, which is different from the metal constituting the base particles (JPP-A-6-228604).
In particular, the aforementioned powder having plural layers of metallic oxide films or metallic films can be imparted with special functions by adjusting the thicknesses of the layers, and for example, in the case where the surface of the base particles is coated with a coated film having a different refractive index with a thickness corresponding to ¼ of the wavelength of the incident light, powder totally reflecting the incident light can be obtained. The inventors have demonstrated that, upon using the powder as base particles of a magnetic material, white toner powder can be produced owing to reflection of light, and furthermore, monochrome powder can also be obtained without use of any dye or pigment by controlling the number of the layers of the optically coherent multilayered film on the surface of the powder and the refractive index and the thickness of each of the layers in such a manner that the coated layers constituting the multilayered film have coherent reflection peaks at the same particular wavelength.
The control for coating the multilayered film is effected by fitting measured values of spectral reflection curves after coating each coated layers to the designed value. The control can be precisely carried out with the plane wave solution of the Maxwell's electromagnetic equation as the designed value in case where the base material is a flat plate. In the case where light having an incident wavelength λ is incident on a multilayer accumulated film having N layers in total at an incident angle ΦN+1, in general, assuming that nj and dj represent the refractive index and the thickness of the j-th layer from the bottom (hereinafter, sometimes referred to as the j-th layer), respectively, and Φj represents the incident angle of the light to the j-th layer, the following recurring formula is obtained by expanding the Maxwell's equation with the amplitude reflection intensity from the j-th layer to the (j+1)-th layer immediately thereon being represented by Rj+1,j.
                                          R                                          j                +                1                            ,              j                                =                                                    r                                                      j                    +                    1                                    ,                  j                                            +                                                R                                      j                    ,                                          j                      -                      1                                                                      ⁢                                  exp                  ⁡                                      (                                                                  -                        2                                            ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                                              δ                        j                                                              )                                                                                      1              +                                                r                                                            j                      +                      1                                        ,                    j                                                  ⁢                                  R                                      j                    ,                                          j                      -                      1                                                                      ⁢                                  exp                  ⁡                                      (                                                                  -                        2                                            ⁢                                                                                          ⁢                      i                      ⁢                                                                                          ⁢                                              δ                        j                                                              )                                                                                      ⁢                                  ⁢                              2            ⁢                                                  ⁢                          δ              j                                =                                                    4                ⁢                                                                  ⁢                π                            λ                        ⁢                          n              j                        ⁢                          d              j                        ⁢            cos            ⁢                                                  ⁢                          ϕ              j                                                          (        1        )            
In the formula, rj+1,j represents the Fresnel reflection coefficient at the interface between the (j+1)-th layer and the j-th layer, the p polarization (with an electric field in parallel to the incident plane) is provided by the following equation,rj+1,j=(nj+1 cosφj−nj cosφj+1)/(nj+1 cosφj+nj cosφj+1)and the s polarization (with an electric field perpendicular to the incident plane) is provided by the following equation.rj+1,j=(2nj+1 cos φj+1)/(nj+1 cos φj+nj cos φj)
The amplitude reflection coefficient Rflat(λ,θ) from the N-layer accumulated film can be obtained by solving the equations.
In case where the base material is powder, however, there arises a problem that even when the coated layers are formed to have such a maximum or minimum reflection wavelength measured with a spectrophotometer that is intended assuming the case where the base material is a flat plate, the multilayered film coated powder finally obtained cannot provide the intended reflection intensity at the intended wavelength.
In order to solve the problem, such a technique has been disclosed that particular compensation is made in the formula providing light reflection from a multilayered film coated flat plate, whereby the coated layers are optimally designed for the thickness thereof providing the maximum or minimum reflection intensity with light having a particular wavelength (for example, see Patent Document 1).
(Patent Document 1)
JP-A-2001-271006
However, the technique disclosed in Patent Document 1 intends only that the reflection peak or bottom of the spectral intensity waveform is positioned in the wavelength range corresponding to the intended color, and the reflectance thereof becomes maximum or minimum. Therefore, powder having a coated structure having been designed according to the technique disclosed in Patent Document 1 has a spectral intensity waveform that is not necessarily agree with the spectral intensity waveform of the intended color. It is necessary that colormatching or toning is carried out by using other pigments or the like to obtain the intended color. However, the conventional organic pigments have a problem of deteriorated weather resistance, which provides color degradation, although brilliant colors can be provided.
Furthermore, in the case where only the reflection peak or bottom is matched, there is such a problem that it provides an unnatural metallic optically coherent color like irisation with high gloss.
Accordingly, an object of the invention is to solve the problems associated with the conventional technique and to provide such optically coherent multilayered film-coated powder that has weather resistance and a brilliant intended color, and processes for designing and producing the same.